are translated, leading to an increase in antenna size. However, 

 if chlorophyll biosynthesis is blocked at an early stage, or cells 

 are placed in the dark, the increased transcript level is not 

 expressed as an increase in the pigment protein complex. The 

 preliminary work on this model system suggests that the 

 transduction signal is processed at the transcriptional level 

 (LaRoche et al., in press). The nature and mechanisms of the 

 controls remain unclear, as does the significance for the reduction 

 of the process in higher plants. Comparative analysis will lead to 

 an understanding of how light intensity signals are perceived and 

 transduced in photosynthetic organisms. 



Pressure Limitation 



Approximately 8 0% of the ocean's volume is below 1000 m in 

 depth, making the high pressures encountered in this habitat the 

 most prevalent in the biosphere. Pressure strongly delineates and 

 limits the distribution of oceanic life, with an influence as 

 significant as that of temperature, salinity, or currents (Yayanos, 

 1986) . Deep-sea organisms, which have evolved specific biochemical 

 and physiological adaptations to deal with hydrostatic pressure, 

 may play a role in the processing and recycling of biogeochemically 

 important compounds (Longhurst and Harrison, 1988) . Additionally, 

 a variety of marine organisms undertake diurnal or ontogenetic 

 migrations between the deep sea and surface waters where they can 

 influence the population structure of surface ecosystems (Ainley et 

 al., 1986; Wakefield and Smith, 1990). The effect of pressure also 

 is important for deep subsurface microbial communities. 



Compared with other physical signals such as light and 

 temperature, relatively little is known about how organisms 

 perceive and respond to changes in hydrostatic pressure. Pressure 

 acclimation is believed to involve alterations in many cellular 

 processes, and appears to be manifested at the level of protein and 

 membrane structure and function. For example, in deep-sea 

 organisms the essential properties of enzyme function, such as 

 ligand binding, catalytic rate, and structural stability, are 

 maintained under pressures which severely perturb these features in 

 non-adapted enzymes from surface-dwelling organisms (Siebenaller 

 and Somero, 1989) . With increasing environmental pressure, the 

 membranes of bacteria and fishes display an increase in the ratio 

 of total unsaturated to total saturated fatty acids, which has been 

 interpreted as a homeoviscous adaptation to maintain membrane 



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